U.S. patent application number 09/984886 was filed with the patent office on 2002-08-01 for optical pickup capable of detecting and/or compensating for spherical aberration.
Invention is credited to Ahn, Young-man, Chung, Chong-sam, Kim, Tae-Kyung, Suh, Hae-jung.
Application Number | 20020101798 09/984886 |
Document ID | / |
Family ID | 19702857 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020101798 |
Kind Code |
A1 |
Kim, Tae-Kyung ; et
al. |
August 1, 2002 |
Optical pickup capable of detecting and/or compensating for
spherical aberration
Abstract
An optical pickup is provided for a recording medium, including:
a light source generating and emitting a light beam; a light beam
division and detection unit dividing a particular light beam
portion of the light beam after being reflected/diffracted from the
recording medium into sub-divided light beams portions, and
detecting the sub-divided light beam portions; and a spherical
aberration detection circuit processing the sub-divided light beam
portions to detect spherical aberration caused by thickness
variation of the recording medium.
Inventors: |
Kim, Tae-Kyung; (Seoul,
KR) ; Ahn, Young-man; (Gyeonggi-do, KR) ; Suh,
Hae-jung; (Gyeonggi-do, KR) ; Chung, Chong-sam;
(Gyeonggi-do, KR) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Family ID: |
19702857 |
Appl. No.: |
09/984886 |
Filed: |
October 31, 2001 |
Current U.S.
Class: |
369/44.23 ;
369/112.12; 369/112.15; G9B/7.102; G9B/7.113; G9B/7.117;
G9B/7.124 |
Current CPC
Class: |
G11B 7/131 20130101;
G11B 7/1381 20130101; G11B 7/1353 20130101; G11B 7/1369 20130101;
G11B 7/007 20130101; G11B 7/00718 20130101; G11B 7/13927
20130101 |
Class at
Publication: |
369/44.23 ;
369/112.15; 369/112.12 |
International
Class: |
G11B 007/095; G11B
007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 2000 |
KR |
2000-74797 |
Claims
What is claimed is:
1. An optical pickup for a recording medium, comprising: a light
source generating and emitting a light beam; an objective lens
focusing the light beam from the light source to form a light spot
on the recording medium; an optical path changer disposed on an
optical path between the light source and the objective lens,
altering a traveling path of the light beam; a light beam division
and detection unit dividing a particular light beam portion of the
light beam passed through the objective lens after being
reflected/diffracted from the recording medium into sub-divided
light beam portions, and detecting the sub-divided light beam
portions; and a spherical aberration detection circuit processing
detection signals resulting from the sub-divided light beam
portions to detect spherical aberration caused by thickness
variation of the recording medium.
2. The optical pickup of claim 1, wherein the light beam division
and detection unit divides the particular light beam portion into a
first light beam portion on an axis crossing an optical axis
parallel to a radial direction or a tangential direction of the
recording medium, and second and third light beam portions, one at
either side of the first light beam portion in the tangential
direction or the radial direction of the recording medium, and
detects a first detection signal from the first light beam portion,
and a second detection signal from the second and third light beam
portions.
3. The optical pickup of claim 2, wherein the spherical aberration
detection circuit detects a spherical aberration signal indicative
of the spherical aberration by subtracting the second detection
signal from the first detection signal, where the spherical
aberration signal is not affected by defocus.
4. The optical pickup of claim 3, wherein a polarity of the
spherical aberration signal is inverted around a point at which a
spherical aberration coefficient is zero, according to a direction
of the spherical aberration, and the spherical aberration signal
has negative values for the spherical aberration in a positive
direction and positive values for the spherical aberration in a
negative direction.
5. The optical pickup of claim 3, wherein the light beam division
and detection unit comprises: a hologram optical element comprising
first, second, and third pattern areas dividing/diffracting the
particular light beam portion into the first, second, and third
light beam portions; and a photodetector unit comprising a first
photodetector receiving the first light beam portion passed through
the first pattern area and outputting the first detection signal,
and a second photodetector receiving the second and third light
beam portions passed through the second and third pattern areas and
outputting the second detection signal.
6. The optical pickup of claim 5, wherein the recording medium
comprises a land-groove structure where the light beam radiated
onto the recording medium is reflected and diffracted into
0.sup.th-order and .+-.1.sup.st-order diffracted light beams, the
.+-.1.sup.st-order diffracted light beams partially overlapping at
an exit pupil of the objective lens, and the particular light beam
portion corresponds to an overlapping portion of the
.+-.1.sup.st-order diffracted light beams.
7. The optical pickup of claim 6, wherein the first, second and
third pattern areas of the hologram optical element divide and
diffract the overlapping light beam portion into the first light
beam portion on the optical axis, and one of the second and third
light beam portions is at either side of the first light beam
portion in the tangential direction or the radial direction of the
recording medium.
8. The optical pickup of claim 7, wherein the recording medium has
a format satisfying: 5 2 xNAxGw < 1where .lambda. denotes a
wavelength of the light source, NA denotes a numerical aperture of
the objective lens, and Gw denotes a groove width of the recording
medium.
9. The optical pickup of claim 5, wherein the photodetector unit
further comprises a third photodetector receiving a remaining light
beam portion passed through the hologram optical element, excluding
the particular light beam portion, and detecting an information
reproduction signal from the recording medium, a focus error
signal, and/or a tracking error signal.
10. The optical pickup of claim 7, wherein the photodetector unit
further comprises a third photodetector receiving a remaining light
beam portion passed through the hologram optical element, excluding
the particular light beam portion, and detecting an information
reproduction signal from the recording medium, a focus error
signal, and/or a tracking error signal.
11. The optical pickup of claim 5, wherein the second and third
pattern areas comprise identical hologram patterns, and the first
pattern area comprises a hologram pattern different from the first
and second pattern areas.
12. The optical pickup of claim 6, wherein the second and third
pattern areas comprise identical hologram patterns, and the first
pattern area comprises a hologram pattern different from the first
and second pattern areas.
13. The optical pickup of claim 3, wherein the recording medium
comprises lands and grooves where the light beam radiated onto the
recording medium is reflected and diffracted into 0.sup.th-order
and .+-.1.sup.st-order diffracted light beams, and light spots of
the .+-.1.sup.st-order diffracted light beams partially overlap at
an exit pupil of the objective lens, and the particular light beam
portion corresponds to an overlapping portion of the
.+-.1.sup.st-order diffracted light beams.
14. The optical pickup of claim 13, wherein the recording medium
has a format satisfying: 6 2 xNAxGw < 1where .lambda. denotes a
wavelength of the light source, NA denotes a numerical aperture of
the objective lens, and Gw denotes a groove width of the recording
medium.
15. The optical pickup of claim 3, wherein the spherical aberration
detection circuit detects the spherical aberration signal by
normalizing a difference signal obtained by subtracting the second
detection signal from the first detection signal, and the light
beam division and detection unit outputs the first and second
detection signals with respect to a sum of the first and second
detection signals.
16. The optical pickup of claim 15, wherein the spherical
aberration detection circuit amplifies at least one of the first
and second detection signals with a predetermined gain factor, and
processes the amplified at least one of the first and second
detection signals to detect the spherical aberration signal.
17. The optical pickup of claim 3, wherein the spherical aberration
detection circuit amplifies at least one of the first and second
detection signals with a predetermined gain factor, and processes
the amplified at least one of the first and second detection
signals to detect the spherical aberration signal.
18. The optical pickup of claim 1, wherein the light beam division
and detection unit detects an information reproduction signal from
the recording medium, a focus error signal, and/or a tracking error
signal, using a detection signal resulting from a remaining light
beam portion of the light beam excluding the particular light beam
portion.
19. The optical pickup of claim 3, wherein the light beam division
and detection unit detects an information reproduction signal from
the recording medium, a focus error signal, and/or a tracking error
signal, using a detection signal resulting from a remaining light
beam portion of the light beam excluding the particular light beam
portion.
20. The optical pickup of claim 1, wherein: the light beam division
and detection unit divides/diffracts the light beam overlapping at
an exit pupil of the objective lens into a first light beam portion
on the optical axis and second and third light beam portions around
the first light beam portion, in an equal area ratio, and detects
first, second, and third detection signals resulting from the
first, second, and third light beam portions, respectively; and the
spherical aberration detection circuit detects a spherical
aberration signal by subtracting the second detection signal
resulting from the second and third light beam portions from the
first detection signal resulting from the first light beam
portion.
21. The optical pickup of claim 1, further comprising a spherical
aberration compensation element on an optical path between the
optical path changer and the objective lens to correct spherical
aberration caused by thickness variation of the recording medium,
wherein the spherical aberration compensation element is driven
according to the spherical aberration detected by the spherical
aberration detection circuit.
22. The optical pickup of claim 21, wherein the spherical
aberration compensation element comprises a liquid crystal plate
comprising two transparent substrates having electrode patterns,
wherein the liquid crystal plate is driven according to the
spherical aberration where a shape of a wavefront of the light beam
is changed into an inverse shape of the spherical aberration to
compensate for the spherical aberration caused by the thickness
variation of the recording medium.
23. The optical pickup of claim 3, further comprising a spherical
aberration compensation element on an optical path between the
optical path changer and the objective lens to correct spherical
aberration caused by thickness variation of the recording medium,
wherein the spherical aberration compensation element is driven
according to the spherical aberration signal detected by the
spherical aberration detection circuit.
24. The optical pickup of claim 23, wherein the spherical
aberration compensation element comprises a liquid crystal plate
comprising two transparent substrates having electrode patterns,
wherein the liquid crystal plate is driven according to the
spherical aberration signal where a shape of a wavefront of the
light beam is changed into an inverse shape of the spherical
aberration to compensate for the spherical aberration caused by the
thickness variation of the recording medium.
25. The optical pickup of claim 1, wherein the light source
comprises a semiconductor laser or a vertical cavity surface
emitting laser.
26. The optical pickup of claim 1, wherein the optical path changer
comprises a beam splitter transmitting and reflecting the incident
light beam in a predetermined ratio.
27. The optical pickup of claim 1, wherein the optical path changer
comprises a combination of a polarizing beam splitter selectively
transmitting or reflecting the light beam according to a
polarization of the light beam, and a quarter-wave plate, between
the polarization beam splitter and the objective lens, changing a
phase of the light beam.
28. The optical pickup of claim 1, wherein the light source
comprises a blue-light semiconductor laser emitting the light beam
having a wavelength of 400-420 nm; and the optical pickup further
comprising: a condensing lens having an NA of at least 0.7 to
record or reproduce on/from a next generation DVD family recording
medium.
29. The optical pickup of claim 1, further comprising: a
collimating lens on the optical path between the light source and
the optical path changer, collimating a diverging light beam from
the light source; and a sensing lens on the optical path on the
optical path between the light source and the optical path changer
and the light beam division and detection unit condensing the light
beam where the light beam is received by the light beam division
and detection unit.
30. The optical pickup of claim 1, wherein the photodetector unit
separately receives the light beam portion excluding the particular
light beam portion from the hologram optical element and the
particular light beam portion and outputs a particular light beam
portion detection signal indicative of the particular light beam
portion to detect a spherical aberration signal, and outputs a rest
of a light beam portion detection signal indicative of the rest of
the light beam portion to detect an information reproduction
signal.
31. The optical pickup of claim 1, wherein the photodetector unit:
separately receives the light beam portion excluding the particular
light beam portion from the hologram optical element and the
particular light beam portion, and outputs a particular light beam
portion detection signal corresponding to the particular light beam
portion indicative of spherical aberration, and outputs a rest of
light beam portion detection signal corresponding to the light beam
portion excluding the particular light beam portion indicative of
an information reproduction signal.
32. The optical pickup of claim 1, wherein the photodetector unit:
receives the particular light beam portion from the hologram
optical element, and outputs a particular light beam portion
detection signal corresponding to the particular light beam portion
indicative of spherical aberration and an information reproduction
signal.
33. The optical pickup of claim 3, wherein the light beam division
and detection unit comprises: a photodetector unit comprising
first, second, and third pattern areas dividing/diffracting the
particular light beam portion into the first, second, and third
light beam portions, a first photodetector receiving the first
light beam portion passed through the first pattern area and
outputting the first detection signal, and a second photodetector
receiving the second and third light beam portions passed through
the second and third pattern areas and outputting the second
detection signal.
34. An optical pickup for a recording medium, comprising: a light
source generating and emitting a light beam; a light beam division
and detection unit dividing a particular light beam portion of the
light beam after being reflected/diffracted from the recording
medium into sub-divided light beams portions, and detecting the
sub-divided light beam portions; and a spherical aberration
detection circuit processing the sub-divided light beam portions to
detect spherical aberration caused by thickness variation of the
recording medium.
35. The optical pickup of claim 34, wherein sub-divided light beam
portions from the light beam division and detection unit comprises
first, second, and third light beam portions, the light beam
division and detection unit comprising: a photodetector unit
comprising first, second, and third pattern areas
dividing/diffracting the particular light beam portion into the
first, second, and third light beam portions, a first photodetector
receiving the first light beam portion passed through the first
pattern area and outputting a first detection signal, and receiving
the second and third light beam portions passed through the second
and third pattern areas and outputting a second detection
signal.
36. The optical pickup of claim 34, wherein the sub-divided light
beam portions from the light beam division and detection unit
comprise first, second, and third light beam portions, the light
beam division and detection unit comprising: a hologram optical
element comprising a first pattern area diffracting/deflecting the
first light beam portion on an optical axis, and second and third
pattern areas diffracting/deflecting the second and third light
beam portions, wherein the second and third pattern areas comprise
a same hologram pattern, and a photodetector unit comprising a
first photodetector receiving the first light beam portion
diffracted/deflected from the first pattern area and a second
photodetector receiving the second and third light beam portions
diffracted/deflected by the first and second pattern areas.
37. The optical pickup of claim 36, wherein the photodetector unit
further comprises a third photodetector receiving a remaining light
beam portion passed through the hologram optical element, excluding
the particular light beam portion, and detecting an information
reproduction signal from the recording medium, a focus error
signal, and/or a tracking error signal.
38. The optical pickup of claim 37, wherein the third photodetector
comprises a divided configuration comprising at least four sections
to detect the focus error signal and/or the tracking error
signal.
39. The optical pickup of claim 36, further comprising: a
condensing lens focusing the second and third light beam portions
on the second photodetector.
40. The optical pickup of claim 36, wherein: the spherical
aberration detection circuit receives a first detection signal Pi
from the first photodetector, and a second detection signal Po from
the second photodetector and normalizes a difference signal (Pi-Po)
of the first and second detection signals Pi and Po with respect to
a sum of the first and second detection signals (Pi+Po) and outputs
a spherical aberration signal corresponding to (Pi-Po)/(Pi+Po),
wherein the spherical aberration signal is not affected by
defocus.
41. The optical pickup of claim 36, further comprising: an
objective lens focusing the light beam from the light source to
form a light spot on the recording medium; and an optical path
changer disposed on an optical path between the light source and
the objective lens, altering a traveling path of the light
beam.
42. The optical pickup of claim 41, wherein the recording medium
comprises a land-groove structure where the light beam radiated
onto the recording medium is reflected and diffracted into
0.sup.th-order and .+-.1.sup.st-order diffracted light beams, the
.+-.1.sup.st-order diffracted light beams partially overlap at an
exit pupil of the objective lens, and the particular light beam
portion corresponds to the overlapping portion of the
.+-.1.sup.st-order diffracted light beams.
43. The optical pickup of claim 42, wherein the recording medium
has a format satisfying: 7 2 xNAxGw < 1where .lambda. denotes a
wavelength of the light source, NA denotes a numerical aperture of
the objective lens, and Gw denotes a groove width of the recording
medium.
44. The optical pickup of claim 36, wherein the second and third
pattern areas comprise identical pattern areas, and the first
pattern area comprises a pattern area different from the first and
second pattern areas.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Korean Application
No. 2000-74797, filed Dec. 8, 2000, in the Korean Industrial
Property Office, the disclosure of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical pickup, and more
particularly, to an optical pickup detecting spherical aberration
caused by thickness deviation of a recording medium, and/or
compensating for spherical aberration caused by the thickness
variation of a recording medium.
[0004] 2. Description of the Related Art
[0005] In general, information recording/reproduction density
increases as a size of a light spot focused on a recording medium
in an optical pickup apparatus becomes smaller. The shorter a
wavelength (.lambda.) of light used and the larger a numerical
aperture (NA) of an objective lens, the smaller the size of a light
spot, which is expressed by equation (1):
size of light spot .alpha. .lambda./NA (1)
[0006] To reduce the size of the light spot focused on the
recording medium in order to obtain a higher recording density,
there is a need to construct an optical pickup with a short
wavelength light source, such as a blue semiconductor laser, and an
objective lens having a larger NA. A format for increasing
recording capacity up to 22.5 GB with a 0.85-NA objective lens, and
for reducing the thickness of a recording medium to 0.1 mm is
desired so as to prevent degradation of performance caused by
tilting of the recording medium. Here, the thickness of the
recording medium is defined as a distance from a light incident
surface of the recording medium to an information recording
surface.
[0007] As shown in equation (2) below, an spherical aberration
W.sub.40d is proportional to a fourth power of the NA of the
objective lens and to a deviation of the thickness of the recording
medium. For this reason, if an objective lens with a high NA of
about 0.85 is adopted, the recording medium must have a
uniform-thickness with a deviation less than .+-.3 .mu.m. However,
it is very difficult to manufacture the recording medium within the
above thickness deviation range. 1 W 40 d = n 2 - 1 8 n 3 ( NA ) 4
d ( 2 )
[0008] FIG. 1 is a graph showing a relation between thickness
deviation of the recording medium and wavefront aberration (optical
path difference (OPD)) caused by a thickness deviation when a
400-nm light source and an objective lens having an NA of 0.85 are
used. As shown in FIG. 1, the wavefront aberration increases
proportionally with the thickness deviation. Thus, when the
objective lens having a high NA, for example, an NA of 0.85, is
adopted, there is a need to correct for spherical aberration caused
by the thickness deviation of the recording medium.
[0009] FIG. 2 shows a conventional optical pickup detecting and
correcting aberration, which is disclosed in Japanese Patent
Publication No. hei 12-155979. Referring to FIG. 2, the
conventional optical pickup includes a light source 10, an
objective lens 17, which focuses a light beam emitted from the
light source 10 onto a recording medium 1, and a half mirror 11
altering a traveling path of the light beam passed through the
objective lens 11 after being reflected from the recording medium
1. A hologram optical element (HOE) 20 divides and deflects an
incident light beam from the half mirror 11 into a light beam
passing through a particular region and a light beam passing
through another region. A photodetector unit 21 includes first
through fourth photodetectors 21a, 21b, 21c, and 21d, which detect
the light beam passed through the particular region (See FIG. 4). A
signal processing circuit 23 detects aberration from the detection
signals of the first through fourth photodetectors, and a wavefront
changing device 25 changes the shape of a wavefront of the light
beam going toward the recording medium 1 from the light beam source
10 according to a signal from the signal processing circuit 23. In
FIG. 2, a collimating lens 13 collimates the light beam emitted and
diverging from the light source 10.
[0010] FIG. 3 illustrates wavefront aberration resulting from
spherical aberration. When spherical aberration occurs, retarded
wavefronts 27a and 27b, which are symmetrical around an optical
axis c, are generated with respect to a reference wavefront 27 at
the aperture center. When spherical aberration occurs, leading
wavefronts, which are symmetrically around the optical axis c, may
be generated.
[0011] As shown in FIG. 4, the HOE 20 includes first and second
diffraction areas 20a and 20b which select, divide and diffract a
retarded wavefront portion such that divided light beam portions
are symmetrical with respect to an x-axis crossing an optical axis
and go toward the first and fourth photodetectors 21a and 21d. The
HOE 20 also includes a third diffraction area 20c, which diffracts
the light beam portion excluding the retarded wavefront portion
above the x-axis such that a diffracted light beam portion goes
toward the second photodetector 21b. A transmission area 20d
transmits the light beam portion below the x-axis such that a
transmitted light beam portion goes toward the third photodetector
21c. The first and second diffraction areas 20a and 20b have a
semicircular shape.
[0012] Each of the first and fourth photodetectors 21a and 21d has
a 2-sectional configuration with which the occurrence of spherical
aberration can be detected by detecting the focus status. Each of
the second and third photodetectors 21b and 21c has a 2-sectional
configuration with which a focus error signal can be detected using
a knife edge method.
[0013] FIGS. 5A through 5C illustrate the variations of light beam
patterns received by the first through fourth photodetectors 21a,
21b, 21c, and 21d according to occurrence of wavefront aberration.
In particular, FIG. 5A illustrates light beam patterns received by
the first through fourth photodetectors 21a, 21b, 21c, and 21d when
a retarded wavefront occurs. Retarded wavefront portions, which are
diffracted by the first and second diffraction areas 20a and 20b of
the HOE 20, are focused behind the first and fourth photodetectors
21a and 21d. The light beam patterns received by the first and
fourth photodetectors 21a and 21d are symmetrical. Relatively
higher amplitude signals are detected by a first section A of the
first photodetector 21a and a second section D of the fourth
photodetector 21d, compared with a second section B of the first
photodetector 21a and a first section C of the fourth photodetector
21d. FIG. 5B illustrates light beam patterns received by the first
through fourth photodetectors 21a, 21b, 21c, and 21d when no
aberration occurs. As shown in FIG. 5B, the first and second
sections A and B of the first photodetector 21a detect signals
having the same magnitude. Also, the first and second sections C
and D of the fourth photodetector 21d detect light signals having
the same amplitude. FIG. 5C illustrates the light beam patterns
received by the first through fourth photodetectors 21a through 21d
when a leading wavefront occurs. In this case, the leading
wavefront portions, which are diffracted by the first and second
diffraction areas 20a and 20b, are focused before the first and
fourth photodetectors 21a and 21d. Relatively higher amplitude
signals are detected by the second section B of the first
photodetector 21a and the first section C of the fourth
photodetector 21d, compared to the first section A of the first
photodetector 21a and the second section D of the fourth
photodetector 21d.
[0014] Thus, a spherical aberration signal SES' is detected by
subtracting a sum of a detection signal b of the second section B
of the first photodetector 21a and a detection signal c of the
first section C of the fourth photodetector 21d, from a sum of a
detection signal a of the first section A of the first
photodetector 21a and a detection signal d of the second section D
of the fourth photodetector 21d, which is expressed as:
SES'=(a+d)-(b+c) (3)
[0015] If this conventional aberration detection method is applied,
both an amount and a polarity of aberration can be detected with
respect to a small amount of spherical aberration. Meanwhile, when
a large amount of spherical aberration occurs due to saturation of
the signal difference, only the polarity of the spherical
aberration, not the amount thereof, can be detected.
[0016] Another drawback of the conventional aberration detection
method lies in that predetermined amplitude of spherical aberration
signal SES' is detected even when only a predetermined amount of
defocus occurs without spherical aberration. Defocus W.sub.20 is
proportional to the square of an NA of an objective lens, which is
expressed as formula (4). Thus, a the degree of retarding and
leading in wavefronts caused by defocus and spherical aberration
differs, but the characteristics of the retarded and leading
wavefronts caused by defocus and spherical aberration are very
similar. 2 W 20 = 1 2 zNA 2 ( 4 )
[0017] where .DELTA.z is the amount of movement of an image
point.
SUMMARY OF THE INVENTION
[0018] Various objects and advantages of the invention will be set
forth in part in the description that follows and, in part, will be
obvious from the description, or may be learned by practice of the
invention.
[0019] To solve the above problems, it is an object of the present
invention to provide an optical pickup accurately detecting
spherical aberration caused by thickness variation of a recording
medium without being affected by defocus, and/or capable of
compensating for such spherical aberration.
[0020] According to an aspect of the present invention, there is
provided an optical pickup for a recording medium, including: a
light source generating and emitting a light beam; an objective
lens focusing the light beam from the light source to form a light
spot on the recording medium; an optical path changer disposed on
an optical path between the light source and the objective lens,
altering a traveling path of the light beam; a light beam division
and detection unit dividing a particular light beam portion of the
light beam passed through the objective lens after being
reflected/diffracted from the recording medium into sub-divided
light beams portions, and detecting the sub-divided light beam
portions; and a spherical aberration detection circuit processing
detection signals resulting from the particular light beam portion
from the light beam division and detection unit to detect spherical
aberration caused by thickness variation of the recording
medium.
[0021] The light beam division and detection unit divides the
particular light beam portion into a first light beam portion on an
axis crossing an optical axis parallel to a radial direction or a
tangential direction of the recording medium, and second and third
light beam portions, one at either side of the first light beam
portion in the tangential direction or the radial direction of the
recording medium, and detects a first detection signal from the
first light beam portion, and a second detection signal from the
second and third light beam portions.
[0022] In this case, the light beam division and detection unit
includes: a hologram optical element comprising first, second, and
third pattern areas dividing/diffracting the particular light beam
portion into the first, second, and third light beam portions; and
a photodetector unit including a first photodetector receiving the
first light beam portion passed through the first pattern area and
outputting the first detection signal, and a second photodetector
receiving the second and third light beam portions passed through
the second and third pattern areas and outputting the second
detection signal.
[0023] The recording medium has a land-groove structure where the
light beam radiated onto the recording medium is reflected and
diffracted into 0.sup.th-order and .+-.1.sup.st-order diffracted
light beams, the .+-.1.sup.st-order diffracted light beams
partially overlapping at an exit pupil of the objective lens, and
where the particular light beam portion corresponds to an
overlapping portion of the .+-.1.sup.st-order diffracted light
beams.
[0024] The recording medium may have a format satisfying the
equation: 3 2 xNAxGw < 1
[0025] where .lambda. denotes a wavelength of the light source, NA
denotes a numerical aperture of the objective lens, and Gw denotes
a groove width of the recording medium.
[0026] The photodetector unit further includes a third
photodetector receiving a remaining light beam portion passed
through the hologram optical element, excluding the particular
light beam portion, and detecting an information reproduction
signal from the recording medium, a focus error signal, and/or a
tracking error signal.
[0027] The optical pickup further includes a spherical aberration
compensation element on an optical path between the optical path
changer and the objective lens to correct spherical aberration
caused by thickness variation of the recording medium, wherein the
spherical aberration compensation element is driven according to a
spherical aberration signal detected by the spherical aberration
detection circuit.
[0028] These together with other objects and advantages, which will
be subsequently apparent, reside in the details of construction and
operation as more fully hereinafter described and claimed,
reference being had to the accompanying drawings forming a part
thereof, wherein like numerals refer to like parts throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The above object and advantages of the present invention
will become more apparent by describing in detail preferred
embodiments thereof with reference to the attached drawings in
which:
[0030] FIG. 1 is a graph showing a relation between a thickness
deviation of a recording medium and wavefront aberration (optical
path difference (OPD)) caused by the thickness deviation;
[0031] FIG. 2 illustrates a conventional optical pickup detecting
and correcting spherical aberration;
[0032] FIG. 3 illustrates wavefront aberration caused by spherical
aberration;
[0033] FIG. 4 illustrates configurations of a hologram and a
photodetector unit of FIG. 2;
[0034] FIGS. 5A illustrates light beam patterns received by the
photodetector unit of when a retarded wavefront occurs;
[0035] FIG. 5B illustrates light beam patterns received by the
photodetector unit when no aberration occurs;
[0036] FIG. 5C illustrates light beam patterns received by the
photodetector unit when a leading wavefront occurs;
[0037] FIG. 6 illustrates an optical arrangement of an embodiment
of an optical pickup according to the present invention;
[0038] FIG. 7 illustrates light beams reflected/diffracted by a
next generation DVD family land/groove type recording medium,
viewed from an exit pupil of an objective lens of the optical
pickup of FIG. 6;
[0039] FIG. 8A illustrates a profile of the reflected/diffracted
light beams of FIG. 7 according to whether aberration occurs;
[0040] FIG. 8B illustrates a magnified view of a profile of the
reflected/diffracted light beams of FIG. 7;
[0041] FIG. 8C illustrates an intensity distribution of an
overlapping light beam portion in tangential and radial
directions;
[0042] FIG. 9A illustrates a profile of the reflected/diffracted
light beams of FIG. 7 from the exit pupil of the objective
lens;
[0043] FIG. 9B illustrates a magnified view of a profile of the
reflected/diffracted light beams of FIG. 7;
[0044] FIG. 9C illustrates an intensity distribution of an
overlapping light beam portion in the tangential and radial
directions;
[0045] FIG. 10A illustrates a profile of the reflected/diffracted
light beams of FIG. 7 from the exit pupil of the objective
lens;
[0046] FIG. 10B illustrates a magnified view of a profile of the
reflected/diffracted light beams of FIG. 7;
[0047] FIG. 10C illustrates an intensity distribution of an
overlapping light beam portion in the tangential and radial
directions;
[0048] FIG. 11 illustrates an example of the light beam divider and
detection unit of the optical pickup of FIG. 6 for a next
generation DVD family land/groove recording medium;
[0049] FIG. 12 is a graph of a spherical aberration signal detected
by a spherical aberration detection circuit of the present
invention with respect to spherical aberration;
[0050] FIG. 13 is a graph of the spherical aberration signal
detected by the spherical aberration detection circuit of the
present invention with respect to defocus;
[0051] FIG. 14A illustrates an intensity distribution of the
reflected/diffracted light beam of FIG. 7 when defocus occurs;
[0052] FIG. 14B illustrates the intensity distribution of the
reflected/diffracted light beam of FIG. 7 when a predetermined
amount of defocus occurs without spherical aberration; and
[0053] FIG. 14C illustrates the intensity distribution of the
reflected/diffracted light beam of FIG. 7 when a predetermined
amount of defocus occurs without spherical aberration.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0054] An optical arrangement of an exemplary embodiment of an
optical pickup according to the present invention is illustrated in
FIG. 6. The optical pickup according to the present invention,
includes a light source 51 which generates and emits a light beam,
an objective lens 57 which focuses an incident light beam emitted
from the light source 51 to form a light spot on a recording medium
50, and an optical path changer disposed on an optical path between
the light source 51 and the objective lens 57, which alters the
traveling path of an incident light beam.
[0055] The photodetector 65 as a light beam division and detection
unit, is constructed such that the photodetector 65 divides and
detects the light beam passed back through the objective lens 57
and the optical path changer after being reflected/diffracted on
the recording medium 50. A spherical aberration detection circuit
70 detects a spherical aberration signal according to thickness
variations of the recording medium 50 by processing a plurality of
detection signals from the light beam division and detection
unit.
[0056] The light source 51 may be a semiconductor laser, such as an
edge emitting laser or a vertical cavity surface emitting laser
(VCSEL). A beam splitter 55 may be used as the optical path
changer, transmitting and reflecting the incident light beam in a
predetermined ratio. Alternatively, the optical path changer may be
a combination of a polarizing beam splitter (not shown), which
selectively transmits or reflects the incident light beam according
to a polarization of the incident light beam, and a quarter-wave
plate (not shown), which changes a phase of the incident light beam
between the polarization beam splitter and the objective lens
57.
[0057] The optical pickup according to the present invention may be
used to record or reproduce on/from a next generation DVD family
recording medium 50 by using a blue-light semiconductor laser as
the light source 51 to emit a beam having a wavelength of 400-420
nm, for example, 405 nm, and a condensing lens as the objective
lens 57 having an NA of 0.7 or greater, for example an NA of
0.85.
[0058] The optical pickup according to the present invention
further includes a collimating lens 53 on the optical path between
the light source 51 and the beam splitter 55, for collimating a
diverging light beam emitted from the light source 51, and a
sensing lens 59 on the optical path between the beam splitter 55
and the photodetector unit 65 for condensing the incident light
beam such that the light beam is received by the photodetector unit
65.
[0059] In an exemplary embodiment, the photodetector unit 65, as
the light beam division and detection unit, divides a particular
light beam portion of the incident beam passed back through the
objective lens 50 after being reflected/diffracted on/from the
recording medium 50 into a plurality of sub-divided light beam
portions, and detects the plurality of the sub-divided light beam
portions, where an intensity distribution of the particular light
beam portion is affected by the thickness variations of the
recording medium.
[0060] In particular, as described below, a variation of the
intensity distribution of the particular light beam portion
according to a direction of spherical aberration is symmetrical
about an axis crossing the optical axis, parallel to a tangential
or radial direction of the recording medium. The intensity
distribution of the particular light beam is inversed according to
the direction of spherical aberration. The light beam division and
detection unit divides the particular light beam portion into a
first light beam portion on the axis crossing the optical axis,
parallel to the radial or tangential direction, and second and
third light beam portions around the first light beam portion in
the tangential or radial direction of the recording medium 50. The
photodetector 65 detects the first, second and third light beam
portions to generate a first detection signal from the first light
beam portion, and a second detection signal from the second and
third light beam portions.
[0061] In an exemplary embodiment of the present invention, the
light beam division and detection unit may include a hologram
optical element (HOE) 61, which divides/diffracts the particular
light beam portion into the first, second and third light beam
portions, and the photodetector unit 65, which receives the first,
second, and third light beam portions divided/diffracted by the HOE
61. The light beam division and detection unit would output the
first detection signal with respect to the first light beam
portion, and the second detection signal with respect to the second
and third light beam portions. The HOE 61 may be arranged between
the beam splitter 55 and the objective lens 57.
[0062] The photodetector unit 65 may be constructed such that the
photodetector unit 65 separately receives the light beam portion
excluding the particular light beam portion from the HOE 61 and the
particular light beam portion. In this case, the photodetector 65
outputs a particular light beam portion detection signal
corresponding to the particular light beam portion, to detect a
spherical aberration signal, and a rest of light beam portion
detection signal output from the photodetector unit 65
corresponding to the light beam portion excluding the particular
light beam portion indicative of an information reproduction
signal. It will be appreciated that an information reproduction
signal may also be detected from the particular light beam portion
detection signal, instead of from the rest of the light beam
portion detection signal. Alternatively, the photodetector unit 65
or another photodetector may be used as the light beam division and
detection unit, with a divided configuration corresponding to the
HOE 61.
[0063] A spherical aberration detection circuit 70 detects a
spherical aberration signal according to the thickness variation of
the recording medium by processing the plurality of detection
signals resulting from the particular light beam portion, which are
output from the light beam division and detection unit. Here, the
spherical aberration signal is not affected by defocus.
[0064] The spherical aberration detection circuit 70 normalizes a
subtraction signal obtained by subtracting the second detection
signal resulting from the second and third light beam portions from
the first detection signal resulting from the first light beam
portion, where the light beam division and detection unit outputs
the first and second detection signals with respect to a sum of the
first and second detection signals, so that a spherical aberration
signal SES is detected. Alternatively, the spherical aberration
detection circuit 70 can be constructed such that the spherical
aberration signal SES is detected by subtracting the second
detection signal from the first detection signal. Here, the
spherical aberration detection circuit 70 may be constructed such
that the spherical aberration signal SES is detected by amplifying
at least one of the first and second detection signals with a
predetermined gain factor and processing the amplified at least one
of the first and second detection signals.
[0065] The optical pickup according to the present invention may
also include a spherical aberration compensation element 75 on the
optical path between the HOE 61 and the objective lens 57 to
compensate for spherical aberration caused by thickness variation
of the recording medium 50 according to the spherical aberration
signal SES detected by the spherical aberration detection circuit
70.
[0066] A liquid crystal plate manufactured by injecting liquid
crystals between two transparent substrates having electrode
patterns may be used as the spherical aberration compensation
element 75. Due to an anisotropic property of the liquid crystal
with respect to a refractive index, a phase of the light beam
passing through the liquid crystal plate changes. In particular,
the liquid crystal plate is driven according to the spherical
aberration signal SES such that the shape of the wavefront of the
incoming light beam is changed into an inverse shape of the
spherical aberration, thereby compensating for the spherical
aberration caused by thickness variation. In this case, a driving
circuit (not shown) driving the spherical aberration compensation
element 75 may be included in or may be separate from the spherical
aberration detection circuit 70.
[0067] The light beam division and detection unit of the optical
pickup according to the present invention, which has the
configuration described above to detect and/or correct spherical
aberration, will be described below with reference to a case using
the next generation DVD family recording medium with lands and
grooves (hereinafter, referred to as "land-groove type recording
medium") as the recording medium 50. The land-groove type recording
medium has a format satisfying the following equation: 4 2 xNAxGw
< 1 ( 5 )
[0068] where .lambda. denotes a wavelength of the light source 51,
NA denotes a numerical aperture of the objective lens 57, and Gw
denotes a groove width of the recording medium.
[0069] The land-groove type recording medium reflects and diffracts
an incident light beam into 0.sup.th order light beam and
.+-.1.sup.st order light beams in a radial direction. As a result,
as shown in FIG. 7, when a light beam LB reflected/diffracted from
the recording medium is viewed from the exit pupil of the objective
lens 57, the .+-.1.sup.st order diffracted light beams partially
overlap.
[0070] FIGS. 8A through 10C illustrate changes of the light beam
deflected/diffracted from the land-groove type recording medium
depending on whether aberration occurs. In particular, FIGS. 8A,
9A, and 10A illustrate profiles of the reflected/diffracted light
beam LB viewed from the exit pupil of the objective lens 57. FIGS.
8B, 9B, and 10B are magnified views of FIGS. 8A, 9A, and 10A,
respectively; illustrating an overlapping portion of the
.+-.1.sup.st order diffracted light beams (hereinafter, referred to
as "overlapping light beam portion"). FIGS. 8C, 9C, and 10C
illustrate an intensity distribution of the overlapping light beam
portion in the tangential and radial directions.
[0071] As shown in FIGS. 8A through 10C, the intensity distribution
of the overlapping light beam portion is almost uniform when no
aberration occurs, i.e., also almost uniform with respect to
defocus, as will be described later. However, when spherical
aberration occurs, the intensity distribution of the overlapping
light beam portion has a Gaussian distribution or an inverse
Gaussian distribution depending on a polarity of aberration.
[0072] In particular, as shown in FIGS. 8A, 8B and 8C, in the case
where no aberration occurs, the intensity distribution of the
overlapping light beam portion is almost uniform. Meanwhile, in a
case where a predetermined amount of spherical aberration occurs,
for example, W.sub.40d=0.9393 .lambda., the overlapping light beam
portion has a Gaussian intensity distribution, as shown in FIGS.
9A, 9B, and 9C, in which a center portion has a peak intensity.
Further, as shown in FIGS. 9A, 9B, and 9C, the intensity of the
light beam exponentially decreases with increased distance from the
center portion. In a case where spherical aberration occurs in a
direction opposite to that of FIGS. 9A, 9B, and 9C, i.e.,
W.sub.40d=-0.9393 .lambda., the overlapping light beam portion has
an inverse Gaussian distribution, in which the center portion has
the lowest intensity and the intensity of the light beam
exponentially increases with increased distance from the center
portion. In other words, the overlapping light beam portion has a
symmetrical intensity distribution around the optical axis, but a
positive spherical aberration and a negative spherical aberration
result in intensity distributions having opposite profiles.
[0073] When the land-groove type recording medium described above
is used as the recording medium 50 and no spherical aberration
occurs, the overlapping light beam portion has a uniform
distribution. Thus, for illustrative purposes, the light beam
division and detection unit divides/diffracts the overlapping light
beam portion into, for example, a first light beam portion P.sub.1
on the optical axis (see FIG. 7), and second and third light beam
portions P.sub.2 and P.sub.3 around the first light beam portion
P.sub.1, in an equal area ratio, and detects signals resulting from
the first, second, and third light beam portions P.sub.1, P.sub.2,
and P.sub.3. The spherical aberration detection circuit 70 is
constructed such that a spherical aberration signal is detected by
subtracting the second detection signal resulting from the second
and third light beam portions P.sub.2 and P.sub.3 from the first
detection signal resulting from the first light beam portion
P.sub.1.
[0074] Referring to FIG. 11, the HOE 61 of the light beam division
and detection unit includes a first pattern area H.sub.1 which
diffracts/deflects the first light beam portion P.sub.1 on an
optical axis c, and second and third pattern areas H.sub.2 and
H.sub.3 which diffract/deflect the second and third light beam
portions P.sub.2 and P.sub.3. In this case, because the second and
third light beam portions P.sub.2 and P.sub.3 have similar
intensity profiles, for illustrative purposes, the second and third
pattern areas H.sub.2 and H.sub.3 are formed having the same
hologram pattern, so that the second and third light beam portions
P.sub.2 and P.sub.3 are received by a single photodetector.
[0075] The photodetector unit 65 includes a first photodetector 65a
receiving the first light beam portion P.sub.1 diffracted/deflected
from the first pattern area H.sub.1. The photodetector unit 65
further includes a second photodetector 65b receiving the second
and third light beam portions P.sub.2 and P.sub.3 focused at the
same location by the condensing lens 59 after being
diffracted/deflected in the same direction by the first and second
pattern areas H.sub.2 and H.sub.3.
[0076] A first detection signal Pi from the first photodetector
65a, and a second detection signal Po from the second photodetector
65b are input to the spherical aberration detection circuit 70. The
spherical aberration detection circuit 70 normalizes, for example,
a difference signal (Pi-Po) of the first and second detection
signals Pi and Po with respect to a sum of the first and second
detection signals (Pi+Po) and outputs a spherical aberration signal
SES expressed as (Pi-Po)/(Pi+Po).
[0077] Although in FIG. 11 the HOE 61 is designed to divide and
diffract the overlapping light beam portion into the first light
beam portion P.sub.1 symmetrically around the optical axis c and
the second and third light beam portions P.sub.2 and P.sub.3
symmetrically around the first light beam portion P.sub.1 in the
tangential direction, and the photodetector unit 65 is designed to
be suitable for the HOE 61, it will be appreciated that the light
beam division and detection unit is not limited to this
configuration. That is, as shown in FIGS. 9C and 10C, because the
intensity distribution of the overlapping light beam portion is
symmetrical around the optical axis in the radial direction as well
as in the tangential direction of recording medium, and because
positive spherical aberration and negative spherical aberration
lead to intensity distributions having opposite profiles, the light
beam division and detection unit of the optical pickup according to
the present invention may be designed such that the overlapping
light beam portion is divided in the radial direction and then
detected by the light beam division and detection unit.
[0078] The optical pickup according to the present invention
including the light beam division and detection unit described
above can detect a spherical aberration signal SES as follows
without being affected by defocus. FIG. 12 is a graph of the
spherical aberration signal SES detected by the spherical
aberration detection circuit 70 with respect to spherical
aberration. Here, a horizontal axis represents a coefficient of
spherical aberration representing an amount of spherical aberration
in units of .lambda.. FIG. 13 is a graph of the spherical
aberration signal SES detected by the spherical aberration
detection circuit 70 with respect to defocus. Here, the horizontal
axis represents a coefficient of defocus representing the amount of
defocus in units of .lambda..
[0079] As shown in FIG. 12, the spherical aberration signal
decreases as the amount of spherical aberration increases from
negative values to positive values. The polarity of the spherical
aberration signal SES is inverted around a point at which a
spherical aberration coefficient is zero, according to a direction
of spherical aberration. The spherical aberration signal SES has
negative values for the spherical aberration in a positive
direction and positive values for the spherical aberration in a
negative direction.
[0080] As shown in FIG. 13, the spherical aberration signal SES is
not affected by defocus. This is also evident in FIGS. 14A through
14C, which illustrate a profile and an intensity distribution of
the light beam reflected/diffracted from the land-groove type
recording medium when a predetermined amount of defocus
(W.sub.20=0.2425.lambda.) occurs without spherical aberration. As
shown in FIGS. 14A through 14C, although a predetermined amount of
defocus (W.sub.20=0.2425.lambda.) occurs, the intensity
distribution of the overlapping light beam portion almost does not
change, unlike when spherical aberration occurs. In other words, if
only defocus occurs without spherical aberration, the intensity of
the overlapping light beam portion is constant in the tangential
direction, but slightly varies in the radial direction. However,
such minor intensity variation in the radial direction is
negligible compared with the intensity variation caused by
spherical aberration. Thus, the intensity distribution of the
overlapping light beam portion is almost uniform in every
direction.
[0081] The offsets of the spherical aberration signal SES when no
spherical aberration occurs as shown in FIG. 12 and when defocus
occurs, as shown in FIG. 13 can be eliminated by dividing the light
beam into the first, second, and third light beam portions P.sub.1,
P.sub.2, and P.sub.3 in an optimal ratio, or by designing the
spherical aberration detection circuit 70 such that the spherical
aberration detection signal SES is detected by amplifying at least
one of the first and second detection signals Pi and Po with a
predetermined gain factor and then processing the detection
signals.
[0082] Therefore, the optical pickup according to the present
invention can accurately detect a spherical aberration signal SES
using the light beam division and detection unit and the spherical
aberration detection circuit 70 described above, without being
affected by defocus. In the present invention, the spherical
aberration signal is detected using a plurality of light beam
portions divided from the particular light beam portion that is
greatly affected by spherical aberration (i.e., the overlapping
light beam portion having .+-.1st order diffracted light beams for
a land-groove type recording medium). Thus, even when spherical
aberration greater than a predetermined amount occurs, the
spherical aberration can be accurately detected without the problem
of signal difference saturation as in a conventional detection
technique.
[0083] Thus, both the amount and the polarity of spherical
aberration caused by thickness variation of the recording medium
can be accurately detected using the light beam division and
detection unit and the spherical aberration detection circuit 70
according to the present invention. In addition, by driving the
spherical aberration compensation element 75 according to the
detected spherical aberration signal SES, spherical aberration
caused by thickness variation of the recording medium 50 can be
corrected.
[0084] Further, the HOE 61 and the photodetector unit 65 can detect
an information reproduction signal from the recording medium 50, a
focus error signal, and/or a tracking error signal, using the
detection signal resulting from the light beam portion excluding
the particular light beam portion. For example, when the
above-described land-groove type recording medium is used as the
recording medium 50, the HOE 61 is designed such that the
particular light beam portion entering through the peripheral area
is transmitted exclusive of the first, second, and third pattern
areas H.sub.1, H.sub.2, and H.sub.3. At the same time, the
photodetector unit 65 is designed such that the photodetector unit
65 further includes a third photodetector 65c, as shown in FIG. 11,
which receives the light beam portion just transmitted through the
HOE 61. In this case, for example, the third photodetector 65c has
a divided configuration including at least four (4) sections to
detect a focus error signal and/or a tracking error signal.
[0085] Although the preferred embodiments of the present invention
are described with reference to the light beam division and
detection unit designed for recording and reproduction on/from a
future generation DVD family land-groove type recording medium, it
will be appreciated that the configuration of the light beam
division and detection unit can be modified for any recording
medium with various formats. The configuration of the optical
pickup according to the present invention of FIG. 6 detecting
and/or correcting spherical aberration caused by thickness
variation of the recording medium 50, is merely illustrative and is
not intended to limit the scope of the present invention.
[0086] As described above, the optical pickup according to the
present invention detects a spherical aberration signal by dividing
a particular light beam portion of a light beam passed through an
objective lens after having been reflected/diffracted from a
recording medium into a plurality of light beam portions, and
detecting the divided light beam portions. The particular light
beam portion is greatly affected by spherical aberration caused by
thickness variation of the recording medium. Thus, spherical
aberration caused by thickness variation of the recording medium
can be accurately detected without being affected by defocus. In
addition, the spherical aberration caused by thickness variation of
a recording medium may be compensated for by driving a spherical
aberration compensation element according to a detected spherical
aberration signal.
[0087] While this invention has been particularly shown and
described with reference to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made thereto without departing from the
spirit and scope of the invention as defined by the appended
claims.
* * * * *